The embodiments herein relate to methods for formulating a cement slurry for use in a subterranean salt formation, and, more particularly, to methods for formulating a cement slurry capable of providing long-term zonal isolation within a subterranean salt formation.
Subterranean formation operations (e.g., stimulation operations, sand control operations, completion operations, etc.) often involve drilling a wellbore in a subterranean formation with a drilling fluid (and thereafter placing a cement sheath between the formation and a casing (or liner string) in the wellbore. The cement sheath is formed by pumping a cement slurry through the bottom of the casing and out through an annulus between the outer casing wall and the formation face of the wellbore. The cement slurry then cures in the annular space, thereby forming a sheath of hardened cement that, inter alia, supports and positions the casing in the wellbore and bonds the exterior surface of the casing to the subterranean formation. This process is referred to as “primary cementing.” Among other things, the cement sheath may keep fresh water reservoirs from becoming contaminated with produced fluids from within the wellbore. As used herein, the term “fluid” refers to liquid phase fluids and gas phase fluids. The cement sheath may also prevent unstable formations from caving in, thereby reducing the chance of a casing collapse and/or stuck drill pipe. Finally, the cement sheath forms a solid barrier to prevent fluid loss or contamination of production zones. The degree of success of a subterranean formation operation involving placement of a cement sheath, therefore, depends, at least in part, upon the successful cementing of the wellbore casing and the cement's ability to maintain zonal isolation of the wellbore.
Subterranean salt formations are often rich in hydrocarbons or other desirable fluids for production to the surface. As used herein, the term “subterranean salt formation” refers to a rock formation composed substantially (i.e., largely but not necessarily wholly) of salt. A variety of salts may be found in a salt formation including, but not limited to, halite, sylvite, bichofite, carnalite, polyhalite, tachydrite, anhydrite, and any combination thereof. However, drilling and cementing in such salt formations may be problematic due to salt creep, for example. As used herein, the term “salt creep” refers to the phenomenon of salt in a formation under stress to deform significantly as a function of time, depending on the loading conditions, and its physical properties, which permits the salt to flow into the wellbore and replace the volume of formation removed by the drill bit. Such replacement may reduce the size of the wellbore and/or may cause the drill pipe to stick and eventually force abandonment of the well. Additionally, during drilling, a drilling fluid may be circulated to and from a wellbore and salt from the formation may become dissolved in the drilling fluid, resulting in, among other things, wellbore opening (i.e., an increase in the radius/diameter of the wellbore), changes in the rheology of the drilling fluid, and the like.
During cementing, the cement slurry may interact with and dissolve at least a portion of the salts in the salt formation, thereby affecting the hydration properties and final cured properties of a cement slurry. For example, dissolution of salt in the cement slurry may influence such cement properties as, without limitation, free fluid, thickening time, compressive strength, rheological properties, and the like. In some cases, the influence of the salt dissolution by changing the geometry of the wellbore and the cement slurry properties may be particularly detrimental and may result in the failure of zonal isolation in a wellbore (e.g., by reducing the wellbore radius and through fluid invasion or other loss of structural integrity to the hydrating or cured cement). Failure of zonal isolation, among other things, may result in environmental contamination, which may cause harm to both flora and fauna, including humans. Such failure may further prevent production or reduce the production capability of a wellbore, which may result in abandonment of the wellbore or costly and time-consuming remedial actions.